Frequently, broadband systems transmit television signals to subscribers of a conditional access system. Broadband systems, such as cable and satellite television systems, typically include a headend for receiving programming, or sessions, and/or services from various sources and redistributing the programming and/or services through a distribution system to subscribers. The headend receives programming signals from a variety of sources, combines the programming signals from the various sources, and transmits the combined signals through the distribution system to subscriber equipment. The distribution system can include a variety of media, such as coaxial cable, fiber optic cable, and satellite links, as well as a network of distributed nodes that then transmit the programming to subscriber locations, or to a network of distributed hubs, which transmit the signals to subscriber equipment, or any combination thereof. In a cable television system, the subscriber equipment can include a cable-ready television, a cable-ready video cassette recorder (VCR), or a digital home communications terminal (DHCT) that is connected to a television, computer, or other display device.
The headend uses modulators to control the streams of data into the distribution system. In today's competitive market, the modulators must be able to accept data/programming from equipment manufactured by many different suppliers. Increasingly, the headend is receiving and transmitting programming in a digital, for example, Moving Pictures Expert Group (MPEG) format, instead of an analog format. Transmitting programs in MPEG format is advantageous because multiple digitized programs can be combined and transmitted in the same 6 MHz of bandwidth that is required to transmit a single analog channel or program.
MPEG transport streams include overhead information such as MPEG tables that indicate the types and location of the programming within the transport stream. In a local television system, the MPEG tables include information that is specific to that local distribution system and its particular channel line-up. MPEG as referenced in this application is described in the MPEG-1 and MPEG-2 standards. The MPEG-1 standards (ISO/IEC 11172) and the MPEG-2 standards (ISO/IEC 13818) are described in detail in the International Organization for Standardization document ISO/IEC JTC1/SC29/WG11 N (June 1996 for MPEG-1 and July 1996 for MPEG-2), which is hereby incorporated by reference. Therefore, the headend system, and the modulators in particular, must add the required MPEG table data to the outgoing bit stream.
Content and data providers provide streams of data, data streams, that include video, audio and data, to cable operators via video sources, such as video encoders and video servers. The data streams are initially prepared for transmission through the broadband system by programming, or mapping, the video, audio and data with control software within a digital network control system (DNCS), which is an element manager for processing data within the headend. The DNCS causes the data streams associated with several programs to be combined into bundled groups of sessions. More specifically, the cable operator defines and maps the specifications of the individual data streams from one or several content and data providers and, for example, multiplexes them into grouped sessions in order to maximize the use of the bandwidth available within the cable television system.
In any broadband system there is a limited amount of bandwidth available. For example, a typical cable television system has a forward bandwidth of 50 Megahertz (MHz) to 870 MHz, which is divided into channels. Therefore, a limited number of modulated channels that can be delivered to a particular DHCT. An example of a modulator is a quadrature amplitude modulation (QAM) modulator that receives a digital bit stream and modulates it for transmission over the cable network. Typically, a channel occupies 6 MHz of bandwidth, and a QAM modulator can generally modulate and transmit data through the bandwidth at a rate of approximately 27 to 38 bits per second depending upon the model of the QAM modulator used. In a typical broadband cable environment, the bandwidth limitation determines the number of services, such as video-on-demand (VOD) and the number of channel offerings that a cable operator may offer its customers.
The modulator modulates the bundled group of sessions with a particular radio frequency (RF), and the modulated signal is provided to the output port of the modulator. A combiner then combines the modulated sessions with other outputs from modulators. The combined modulated outputs are then provided downstream via the distribution network to a plurality of DHCTs. There are numerous bundled groups of sessions that can be programmed by the DNCS and provided to numerous modulators; however, each bundled group is modulated with a different frequency across all the modulators.